1. Field of the Invention
This invention relates to a joint part, such as a connecting pipe or a connecting valve, attached to a resin fuel tank.
2. Description of the Art
As a joint part such as a connecting pipe or a connecting valve, attached to a rim of an opening of an automotive resin fuel tank, resin-made joint parts have been recently employed. Generally, such a resin-made joint part is attached to a resin fuel tank by means of heat plate welding.
Such a resin fuel tank generally has a multi-layer structure containing a low fuel permeation layer formed of a material capable of lowering fuel permeation, such as an ethylene vinyl alcohol copolymer (EVOH), in consideration of prevention of evaporative emission, and an outermost layer formed of, for example, high density polyethylene (HDPE) for impact resistance, chemical resistance, water resistance, economical reasons and the like.
As a joint part such as the above-mentioned connecting pipe, a low fuel permeation material, such as polyamide 12 (PA12), is employed. However, such a joint part made of the PA12 does not adhere well to the outermost layer made of HDPE or the like of the resin fuel tank. Therefore, there has been proposed a joint part including a welding member 60 interposed and welded between a joint part 50 and an outermost layer of a resin fuel tank T, as shown in FIG. 4 (see Japanese Patent No. 2715870). The welding member 60 is made of polyethylene resin such as modified polyethylene, which is strongly weldable to both of such a joint part and such an outermost layer.
It is necessary that a certain thickness of the above-mentioned welding member 60 should be maintained after the welding member is welded therebetween in terms of welding accuracy. Further, the welding member 60 has an insufficient effect of preventing permeation (evaporative emission) of gasoline, especially, gasohol (alcohol-containing gasoline). In other words, polyethylene resin, such as modified polyethylene, as a material for forming the welding member 60, has sufficient weldability, however, a low barrier effect against fuel. For this reason, such a joint part has a defect that fuel may permeate through the welding member 60 and thus evaporate to the outside.